With the progress of computerization in recent years, even portable information terminals are required to have a processing capacity comparable to that of a personal computer in the past. In line with this trend, there is also a demand for video display devices with high definition, high quality and preferably with low-profile, light weight, wide viewing angle and low power consumption.
In response to these requests, a display device with thin-film active elements (thin-film transistor, simply referred to as “TFT”) formed on a glass substrate in matrix form and electro-optic elements formed thereon is being actively developed.
The mainstream of the substrates on which active elements are formed is one with a semiconductor film of amorphous silicon or poly-silicon, etc., formed, patterned and connected with metal wires. Due to differences in electrical characteristics of active elements, the former requires a driving IC (Integrated Circuit) and the latter features the ability to allow a drive circuit to be formed on the substrate.
While the former, the amorphous silicon type, is popular for large liquid crystal displays (simply referred to as “LCD”) currently being widely used, the latter, the poly-silicon type, is becoming the mainstream for medium or small liquid crystal displays.
Only poly-silicon type electro-luminescence type (organic EL) displays featuring self-light-emission, thin, lightweight and wide view-angle are being mass-produced.
An organic EL element is generally combined with a TFT and a current is controlled using a voltage/current control action thereof. Here, the voltage/current control action refers to an action of controlling a current between the source and drain by applying a voltage to the gate terminal of the TFT. By so doing, it is possible to adjust light-emitting intensity and display desired gradation.
The use of such a structure, however, causes the light-emitting intensity of the organic EL element to be quite sensitive to being affected by TFT characteristics. In particular, poly-silicon TFT, poly-silicon TFT formed in a low-temperature process called “low-temperature poly-silicon” is above all confirmed to generate relatively large differences in electrical characteristics between adjoining pixels, which constitutes one of the major causes for the deterioration of the display quality of the organic EL display, particularly display uniformity in the screen.
As shown in FIG. 12, the prior art discloses means for correcting a threshold voltage of a poly-silicon TFT 365 which drives an organic EL element.
With an illumination line 340 and auto-zero illumination line 330 set to L levels to turn ON TFT 375 and TFT 370, a select line 320 is set to L level to set a data line 310 to a reference voltage which is higher than a maximum voltage of a data signal. In this way, the gate voltage of a TFT 365 is set to a threshold voltage of the TFT 365. As a result, the difference between a threshold voltage Vth and the reference voltage is charged in a capacitance 350 and the difference between the threshold voltage Vth and supply voltage+Vdd is charged in a capacitance 355.
Next, the illumination line 340 and auto-zero illumination line 330 are set to H level to turn OFF the TFT 375 and TFT 370 and the data signal is set in the data line 340 in this condition. This causes the gate voltage of the TFT 365 to be shifted. This gate voltage corresponds to the threshold voltage of the TFT 365 and this gate voltage can compensate for the threshold voltage of the TFT 365 for each pixel.
Then, the illumination line 340 is set to L level to turn ON the TFT 375, a current corresponding to the gate voltage to which the TFT 365 is set is supplied to an OLED 380 and the OLED 380 emits light. Furthermore, even after the select line 320 is set to H level, the gate voltage of the TFT 365 is kept to the same voltage and the current corresponding to the data signal flows into the OLED 380.
That is, in the prior art shown in FIG. 12, a potential Vg applied to the gate terminal of the TFT 365 is expressed by Vg=Vth+Vd*Cc/(Cc+Cs), where Vth is the threshold voltage of the TFT 365, Vd is a gradation voltage and Cc, Cs are capacitance values shown in FIG. 12. Thus, since the threshold voltage Vth of the TFT 365 of each pixel is always added to Vg, it is possible to give an offset to Vg without changing the gradation voltage Vd even if Vth differs from one pixel to another.